This invention relates to an improved apparatus for the detection of radon gas using a track registration material.
A known technique employed in the field of radon gas detection which has been found to be highly effective is the method and apparatus for detecting radon and alpha emitting daughters of radon disclosed and claimed in U.S. Pat. No. 3,665,194 to Alter, et al. for "Radon Detection", the disclosure of which is hereby incorporated by reference. Briefly, according to this technique, a strip of track registration material employed as an alpha particle detector is disposed in a protective environment at a preselected location in order to monitor the presence of radon and alpha emitting daughters of radon. If the track registration material is irradiated by alpha particles falling within the sensitivity range of the track registration material, minute damage "tracks" are created therein, which tracks can be subsequently enlarged and made visible under suitable magnification by contact with a reagent to which the tracks display preferential chemical reactivity.
Typically, this radon monitoring technique is carried out by placing one or more housings, each containing a strip of track registration material, in locations to be monitored (such as a mine or a dwelling), and permitting the detectors to remain at the monitored site for a preselected period of time e.g. several weeks). After exposure, the housings are removed, each strip of track registration material is subjected to a chemical etching solution, and the number of tracks etched per unit area on each strip is counted by microscopic inspection. The thus-obtained track data is then used to compute the average radon concentration in the monitored site.
While the above-described technique has been primarily employed in the past for uranium prospecting applications, more recently this technique has been used to measure radon anomalies in earthquake prone areas, to measure radon levels in housing structures located in regions suspected of exhibiting abnormal levels of radon concentration, and as a personal dosimeter to measure the degree of exposure of personnel employed in the mining industry. The most popular track registration type detector employed in the past comprises a housing resembling a plastic drinking cup and a strip of track registration material adhered to the inside bottom surface of the cup at a prescribed distance away from the mouth of the cup, the distance being greater than the transit range of alpha particles in the atmosphere, so that the material is exposed to alpha particles emitted by only gaseous sources migrating into the interior of the housing. Since the range in air of radon-related alpha particles is typically no more than about 8.5 centimeters, this geometry imposes a minimum dimensional constraint on the size of the housing, which has impaired the suitability of such detectors for structural survey applications, particularly for rooms in dwellings and for personal dosimeter applications. Specifically, in structural survey applications the cup must be attached to a wall of the structure or suspended from the ceiling, each of which is at best awkward with a cup of minimum dimensions as noted. In addition, the adverse aesthetic effects inherent with the use of such a detector militate against their acceptability in household and office applications. In personal dosimeter applications, the housing is too large to be conveniently attached to the clothing of the user's, and thus must be incorporated into the users equipment, such as a mining helmet or the like.
As a result of the limitations noted above, recent efforts have been directed toward designing special track registration detectors dedicated to a particular application. For example, in commonly assigned U.S. Pat. No. 4,338,523 issued July 6, 1982, for "Low-Cost Track Registration Radiation Detector", a track registration detector is disclosed which is designed specifically for use in monitoring radon levels in interior structures, particularly personal dwellings. While this device is eminently suited for structural survey uses, use of the same device alone in ground survey applications would not yield useful results due to the physical proximity to sources of alpha emitting particles having energy within the susceptability range of the track registration material employed. Similarly, use of the device shown in the above-noted U.S. patent in personal dosimetry applications would require such precautionary measures to avoid closely adjacent sources of alpha radiation that it would be difficult to obtain reliable track registration data. Thus, a need has existed for a low cost track registration type alpha particle detector which can be used effectively and interchangeably in a wide variety of essentially different types of radon monitoring applications.
One impediment to the design of such a universal track registration type detector has been the conflicting dimensional constraints involved. On the one hand, structural survey and personal dosimeter applications require housings having extremely small size so as to be convenient to emplace on a wall or carry about the person; on the other hand, for most accurate results it has been thought necessary until recently to locate the track registration material in the housing at the minimum distance noted above (8.5 centimeters) from the nearest point of entry into the housing. In addition, the design philosophy has favored placement of the track registration material the same minimum separation distance away from surfaces on which alpha-emitting radon daughter products can plate out, in order to limit detection to alpha particles emanating from gaseous sources only. This additional minimum dimensional constraint has prevented the development of track registration detectors having the reliability and accuracy of standard cup size detectors, while at the same time having physical dimensions small enough to promote use of the detector in the personal dosimeter and structural survey applications.
One solution proposed to the above dilemma has been a relatively small radon diffusion chamber closed at the top by a fiberglass filter and containing two detector strips, one at the bottom of the chamber and another in front of the filter. The combination of the fiberglass filter and a special cover design theoretically traps all aerosols and radon daughter products so that only a gaseous mixture of radon/thoron diffuses through the filter and through holes in the perforated detector foil in the front of the detector into the sensitive inner volume of the chamber. In principle, the foil detectors principally register the alpha particles from radon decay products plated out on the surface of the chamber and also alpha particles present in the air which diffuses through the filter. By using preferential etching techniques, only alpha particles having energies between 0.5 and 2.0 MEV are detected.
While experimentally useful, the above proposed diffusion chamber suffers from several disadvantages. Firstly, the design requires careful assembly so that the detector foils are properly placed in special recesses in the conically shaped wall of the chamber. Secondly, the experimental diffusion chamber has no provision for mounting the device out of the reach of children so that it is subject to being tampered with, which would defeat the purpose of the detector. In addition, the foil detector is not only sensitive to alpha particles emanating from the gases which enter the interior volume and the radon daughter products produced within the volume, but also to alpha particles emitted by substances within the housing material itself.